Open-cell porous bodies can be used as filters in a variety of applications. Very fine porosity is desired for chemical processing, pharmaceutical processing, refining waste water, purifying foods and energy production to name a few. The filters employed in these processes are used to purify, concentrate, sterilize and separate materials. The listed applications require filters with pores sizes in the ultrafiltration (100 nm to 10 nm) and nanofiltration (10 nm to 1 nm) ranges. Some applications require the filters survive thermal excursions in excess of 1500° C., such as in high temperature gas mixing. Typically filters having pores in the low end of ultrafiltration and into the nanofiltration range have required the use of thin membrane (e.g. polymers) of nano-sized pores supported on a substrate (e.g. metal, polymer, ceramic) having larger pores. In general filters in the nanofiltration range are complex to manufacture, do not sustain significant amounts of open porosity during high thermal excursions and are difficult to reproduce in 3-dimensional structures.
U.S. Pat. No. 6,565,825 to Ohji, which is herein incorporated by reference, has shown that alumina powders can be sintered to form porous alumina structures. However, sintering temperatures in excess of 1250° C. reduce porosities to 36 volume percent or below. Ohji further shows that combining alumina hydroxide Al(OH)3 with the alumina powder, and then subsequently sintering, can transform the Al(OH)3 through γ→θ→α phases to provide materials that maintain porosities of 36 volume percent up to 1250° C.
The present invention aims to eliminate the need for a membrane supported by a substrate and provide nano-sized open-cell porosity that can be scaled to any 3-dimensional structure. The present invention also aims to improve the thermal stability of highly porous materials to beyond 1500° C.